1. Field of the Invention
The present invention is directed to an ink-jet pen that has a rectangular ink pipe and mesh filter that extend into compressive contact with a body of resilient synthetic foam. The corners of the ink pipe provide a capillary fluid path for ink past air bubbles that may form in the ink pipe.
2. Description of the Related Art
Many ink-jet printers employ disposable print cartridges or "pens." Ink jet pens have a printhead and a connected ink chamber filled with a supply of ink. The printhead is a sophisticated micromechanical part that contains an array of either thermal resistors or piezoelectric transducers that are energized to eject small droplets of ink out of an array of miniature nozzles.
The ink in the pen must be held in the ink chamber at less than atmospheric pressure so that it does not drool out of the nozzles. However, this negative relative pressure, or backpressure, must not be so great that air is gulped from outside of the printhead through the nozzles and into the interior of the firing chambers. If air gets into the printhead ink channels or firing chambers they "deprime" and no longer function.
U.S. Pat. No. 4,771,295 (Baker et al.), which is assigned to Hewlett-Packard Company (HP), the assignee of the present invention, discloses an ink-jet pen that uses synthetic foam for ink retention and backpressure. Ink is held in the foam at the appropriate backpressure by capillary action. A key feature of the pen disclosed in Baker '295 is a circular ink pipe that extends upward from a bottom wall of the pen body and into compressive contact with the foam. The ink pipe is the fluid conduit for the ink from the foam to the printhead. A wire screen or filter is mounted to the top of the ink pipe. The ink pipe and screen locally compresses the foam to thereby increase its capillarity in the region of the ink pipe. As ink is depleted from the foam, the increased capillarity near the ink pipe tends to draw ink from all other portions of the foam toward the ink pipe, so that the maximum amount of ink can be drawn from the foam for printing.
It is important in such foam-based pens to keep the ink pipe in secure sealing contact with the foam. Ink is held in the ink pipe at less than atmospheric pressure. The opening of the ink pipe that is in contact with the foam functions in conjunction with the ink to provide a gasket-like seal. If this seal is broken and an air path forms from the ambient air into the interior of the ink pipe, the ink pipe will ingest air and the backpressure will be lost, resulting in a catastrophic deprime of the pen. The opening of the ink pipe of previous-generation foam-based pens of the assignee have had circular cross sections. A circular opening provides a smooth and uniform sealing surface and a uniform compression with the foam around its perimeter.
A certain amount of air is dissolved in the liquid ink, which is typically water based. Some amount of this air will leave solution and will collect as bubbles, particularly if the temperature of the ink is increased. Air in the main ink chamber that comes out of solution is either trapped in the foam or escapes to the outside of the pen. In either case, no damage is done. However, if air in the ink pipe comes out of solution, it will be trapped in the ink pipe. Once the filter is wet, its bubble pressure precludes ink from passing from the ink pipe into the main ink chamber. And because the ink pipe is typically close to the printhead, the ink in the ink pipe has a tendency to heat up slightly during printing, causing dissolved air to leave solution. Over time, since the ink in the ink pipe in being replaced by ink from the foam, a continuing amount of air that leaves the ink solution will accumulate as a bubble in the ink pipe. In addition, a certain amount of air may be gulped into the ink pipe from the printhead.
Air bubbles tend to form a generally spherical shape. Since the ink pipe in HP's previous-generation foam-based pens are circular, if the bubble gets large enough, it can extend across the entire ink pipe and can block fluid flow, somewhat like a check ball. This is particularly a problem in pens that are used in the printer with the ink pipe oriented vertically, since the bubble naturally rises and will accumulated at the top of the ink pipe and extending completely across the ink pipe. This bubble can therefore preclude ink from entering the ink pipe. If this happens, the printhead will be starved of ink and the nozzles will deprime.
With circular cross-section ink pipes, an approach of solving the bubble occlusion problem is to form narrow capillary grooves along the longitudinal axis of the ink pipe. However, forming such grooves is difficult and adds risk to the molding process, because such small grooves are areas where molding parts can stick and cause problems, including damage to the molded part.
In addition, circular mesh filters inherently produce waste of the filter material. These filters are formed of sheets of stainless steel mesh, which is relatively expensive. The circular pieces must be discarded. It would be preferable to provide a filter that did not result in such waste of filter materials.
Thus, there remains a need for an ink-jet pen having an ink pipe that forms a positive seal with the foam and yet which does not allow for accumulated air bubbles to form occlude the ink pipe and therefore preclude ink flow. Preferably this pen would also would be easily moldable and avoid the waste of materials inherent with circular filters.